Protective coatings are applied to the interior of metal food and beverage containers (e.g. cans) to prevent the contents from coming into contact with the metal surfaces of the containers. Contact of the container contents with the interior metal surface, especially where acidic products such as soft drinks and tomato juice are involved, can lead to corrosion of the metal container and result in contamination and deterioration of the contents. Protective coatings are also applied to the interior of food and beverage containers to prevent corrosion in the headspace of the container between the fill line of the food product and the container lid, which is particularly problematic with high salt content food products.
Metal container interiors are typically coated with a thin thermoset film to protect the interior metal surface from its contents. Various natural and synthetic resins and their blends have been used as interior can protective coatings, including poly(vinyl chloride) resins, epoxy-functional resins, alkyd/aminoplast resins and oleoresinous materials. These heat-curable compositions are usually applied as solutions or dispersions of one or more resin in volatile organic solvents.
The heat-cured protective coating compositions generally should exhibit sufficient adhesion and flexibility to maintain film integrity during container fabrication. Sufficient coating adhesion and flexibility also are needed for the coating composition to withstand processing conditions the container is subject to during product packaging. Other desired performance features of the cured coatings include corrosion protection and sufficient chemical, abrasion and mar resistance. The coatings used on the interior of metal food containers preferably also meet government regulatory criteria.
Multi-coat coating systems recently have been used to coat the interior of food and beverage containers, wherein the over-coat or lacquer contains an epoxy resin cross-linked with a phenolic resin. Such epoxy-based “Gold lacquers” typically exhibit good adhesion and can be used for storing acidic foodstuffs and beverages. However, there is a perception that some epoxy-based coatings; containing mobile Bisphenol A (BPA), Bisphenol F (BPF), Bisphenol A diglycidyl ether (BADGE) and Bisphenol F diglycidyl ether (BFDGE); are less desirable for foodstuffs storage.
Currently, the food packaging industry and consumer groups are seeking coated metal packaging articles prepared from coating compositions free from mobile BPA, BPF, BADGE and BFDGE; exhibiting excellent corrosion and chemical resistance; and acceptable adhesion and flexibility during container fabrication. The art continues to seek an ideal coating composition for use as a protective coating for metal foodstuffs containers.